Vehicle automatic transmissions include a plurality of annular pistons acted upon by fluid pressure to produce the desired sequence of gear selection during the transmission operation. In the past, automatic transmission pistons are usually formed as die castings or as multiple piece stampings wherein the components are welded or otherwise assembled.
Pistons formed by die casting may be accurately constructed, but the cost is high and porosity problems are often encountered. Automatic transmission pistons constructed from a plurality of stamped sheet metal components may take a variety of configurations, but this mode of construction is also troublesome in that the stamped components must be very accurately assembled and welded together and problems of reliability and cost exists.
It is an object of the invention to provide a piston for vehicle automatic transmissions which is of a simple one piece stamped sheet metal construction which may be economically and accurately fabricated, and is reliable in operation.
A further object of the invention is to provide an automatic transmission piston of stamped construction wherein a seal receiving groove is rolled within the piston flange, and yet, reduced radial wall thickness of the flange is avoided.
A further object of the invention is to provide a method for forming a one piece stamped sheet metal automatic transmission piston having a peripheral seal receiving groove defined within a piston flange wherein the flange configuration is formed by rolling without a significant reduction in the radial thickness of the flange occurring.
An additional object of the invention is to provide a method for forming an automatic transmission piston of one piece sheet metal stamped construction wherein the piston includes a flange having a rolled seal receiving groove defined therein and wherein metal radially displaced during the rolling is provided by the flow of flange material produced by axially compressing the flange during rolling.
In the practice of the invention a cup-shaped annular sheet metal blank is formed by a stamping process. The blank includes a radially extending pressure face and an axially extending flange of cylindrical configuration. The axial length of the flange blank is greater than the flange axial length desired in the finished piston.
The piston blank is placed within a fixture capable of rotating about an axis coinciding with the piston axis. The fixture radially supports the piston flange inner surface. A mating fixture, also capable of rotating about an axis coinciding with the piston axis, is capable of moving parallel to the piston axis so as to apply force upon the piston flange for its entire circumference. At least one roller die and slide is mounted adjacent to the retained blank, having an axis of rotation parallel to the piston and fixtures. The roller die is mounted on a slide capable of moving radially with respect to the axis of rotation of the piston and fixtures. The roller peripheral shape corresponds to the form to be rolled into the piston flange to produce the seal receiving groove and flange configuration desired.
As the piston blank is of a greater initial axial dimension than desired in the finished product the flange is axially compressed by the associated fixture simultaneously during the flange rolling. The metal radially displaced outwardly due to the rolling is primarily supplied by the metal flow resulting from the flange axial length reduction, and accordingly, during flange rolling, and in the finished product, the minimum radial thickness of the flange continuously substantially corresponds to the original flange radial thickness avoiding thinning and attendant localized stress. Preferably, the metal displaced during the rolling of the flange is supplied at a rate substantially equal to the rate that metal is available due to the axial length reduction of the flange and the simultaneous axial and radial flow of metal within the flange produces a homogeneous flange construction having smooth uninterrupted metal flow contours relatively free of stress resulting in a high strength automatic transmission piston of high reliability at reduced cost.